REFLEX REGULATION OF EXPIRATORY MOTOR ACTIVITY

The general, long-tern objective of this proposal is to improve our
understanding of the role of the expiratory muscles in the control of
ventilation. The importance of the expiratory muscles on regulating
expiratory airflow and the end expiratory lung volume is well known, but
our understanding of their neural control is limited. We have learned from
prior studies that lung inflation activities the abdominal and internal
intercostal expiratory muscles through the influence of vagal afferent, but
the role played by chemoreceptor in the control of these muscles is
uncertain. Recent studies in a variety of species have shown that steady
state hypoxia causes a diminution of abdominal and internal intercostal
motor activity, particularly after vagotomy. In the present proposal, the
vagotomized cat preparation will be utilized to explore the mechanisms
underlying this effect. The studies are designed to answer three major
questions: 1) Is the reduction in expiratory motor activity during hypoxia
mediated by inhibitory inputs from the carotid body? This question will be
evaluated by comparing changes in the efferent expiratory activity of
abdominal and internal intercostal muscles and motor nerves induced by
sustained electrical stimulation of a carotid sinus nerve, with that
induced by steady state isocapnic hypoxia; 2) Does the inhibitory influence
exert its effect ont he expiratory pre-motor neurons in the brain stem, or
at the spinal core level? This question will be addressed by analyzing
simultaneously the activity of abdominal muscle pre-motor neurons, and the
efferent activity of motor nerves to the abdominal and internal intercostal
muscles during steady state isocapnic hypoxia; 3) Are changes in the
compound activity recorded in whole abdominal and internal intercostal
nerves during hypoxia due to changes in motoneuron firing rate, recruitment
pattern, or to a combination of both mechanisms? This question will be
evaluated by analyzing changes in the activity of individual abdominal and
internal intercostal muscle motoneurons during isocapnic hypoxia. The
remaining experiment will examine how changes in the duration central
expiratory period and its subdivisions influence the intensity of discharge
in the motor nerves to the abdominal and internal intercostal muscles.
Knowledge gained from the above studies will lead to a comprehensive
understanding of the physiology of the expiratory muscles. The control of
expiratory muscles by central nervous system is of obvious importance in
obstructive lung disease, exercise, and other conditions where the pathway
for expiratory airflow is impeded.